Cutting inserts suffer from a limited service life in peripheral rotary milling applications, especially when machining difficult-to-machine materials. Difficult-to-machine materials include, for example, specialty metals such as titanium and titanium alloys, nickel and nickel alloys, superalloys, and certain exotic metals. Cutting inserts comprising a positive rake face geometry on both the axial cutting face and the radial cutting face are commonly employed in milling operations involving the use of a peripheral rotary tool holder with an indexable capability. The positive cutting geometry of the inserts reduces the cutting forces and consequently reduces power consumption, resulting in a more efficient milling operation. In addition, the cutting inserts typically used in peripheral rotary milling are generally parallelogram-shaped (i.e., each has a generally parallelogram-shaped profile when viewed from a point above the insert's top surface), with two long sides forming two main cutting edges and two short sides forming two minor cutting edges. These types of cutting inserts provide more efficient machining by providing the capability of a larger depth of cut, though such inserts are not as strong as square-shaped cutting inserts.
European Patent No. 0 239 045 provides a parallelogram-shaped cutting insert having a constant positive radial rake angle and a constant radial clearance angle along the major cutting edges.
U.S. Pat. No. 5,071,292 describes a parallelogram-shaped cutting insert having a continuous curved radial cutting face and radial clearance face wherein both the radial rake angle and the radial clearance angle remain substantially the same along the main cutting edge with respect to the associated cutter or tool holder.
U.S. Pat. No. 5,052,863 provides a method for securely locating a parallelogram-shaped cutting insert having a relatively large positive radial clearance angle along the main cutting edge in a tool holder. The method involves adapting a tool holder designed to accommodate an insert having a lower radial clearance angle, to overcome the strength problems associated with greater unsupported overhang when using the parallelogram-shaped cutting inserts having larger radial clearance angle.
U.S. Pat. No. 5,388,932 describes an angled chamfer at the elevated corner nose area of a parallelogram-shaped cutting insert, wherein the angled chamfer increases the cutting edge strength at the main corner nose while maintaining a positive radial rake angle along the main cutting edge.
U.S. Pat. No. 6,142,716 also describes an angled chamfer with a positive radial rake angle, but further comprises a recess at the major cutting sides enabling more rigid localization of the cutting insert in the tool holder and use of less material in manufacturing the cutting insert.
Efforts in the industry to develop new or improved parallelogram-shaped cutting inserts have been directed toward achieving reduced cutting forces, reduced power consumption, increased cutting edge strength, and increased tool life. From the point view of geometrical design, maintaining a positive or a positive plus constant radial rake angle along the main cutting edge has been a fundamental goal of these efforts.
The position of the cutting insert in the associated tool holder may also contribute to achieving the goals of reducing cutting forces and increasing cutting edge strength. Known patent publications and published literature regarding parallelogram-shaped cutting inserts including those described above do not recognize a quantitative relationship between the cutting insert geometry and the position of the cutting insert in the associated tool holder.
Therefore, there is a need for an improved parallelogram-shaped cutting insert and for a milling cutting tool system including plural cutting inserts and a tool holder providing a more efficient and more effective method for machining difficult-to-machine materials.